Industrial linear actuators perform a variety of functions, such as linearly translating a locating pin, or operating a clamp for maintaining a position of a workpiece. A typical linear actuator comprises a housing having a linearly-translating shaft that is operably coupled to a drive means, such as a pneumatic piston and cylinder arrangement, or a geared electric motor. In many applications, precise positioning of the linearly-translating shaft is essential to maintaining specific tolerances in a final assembly of the workpiece.
It is often desirable that the shaft of the linear actuator not rotate with respect to the housing, but rather, extend in a straight line along a single axis without rotation about the axis. Thus, it is desirable that the yaw, pitch, and roll of the shaft with respect to the linear translation be minimized. Accordingly, attempts have been made to accurately position the shaft with respect to the housing, wherein various mechanisms and shaft designs have been used to prevent such yaw, pitch, and roll. One common example is illustrated in FIG. 1, wherein a conventional linear actuator 10 is provided having a square shaft 15 that extends and retracts with respect to a housing 20 for positioning a workpiece (not shown). The housing 20, is provided with a square bore 25, wherein the square bore, in conjunction with a sacrificial square bearing 30, guides the shaft 15 throughout its extension and retraction. The sacrificial square bearing 30 is typically comprised of a material that is substantially softer than the square shaft 15, thus allowing the square bearing to wear more quickly than the typically more-expensive square shaft.
The implementation of a sacrificial square bearing 30, however, typically requires the sacrificial square bearing to be replaced on a regular basis, thus leading to increased maintenance costs. Further, while the square shaft 15 and square bore 25 may last significantly longer without requiring replacement than the sacrificial square bearing 30, tight dimensional tolerances of the bearing surfaces 35 of square shaft 15, square bore 25, and square bearing 30 are still typically maintained for accurate operation of the linear actuator. Accordingly, dimensions of twelve or more bearing surfaces that are present between the square shaft 15 and the square bore 25 and square bearing 30 are typically held tightly during the manufacture of the linear actuator 10.
If manufacturing tolerances are not tightly held between the square shaft 15, the square bore 25, and the sacrificial square bearing 30, a potential pitch, yaw, and roll of the square shaft 15 with respect to the housing 20 can present itself, due to increased slop between the shaft, the square bore, and the square bearing. Inaccuracies in positioning of the square shaft 15 with respect to the housing 20 further tend to increase as the usage of the linear actuator 10 increases, thus leading to an even greater potential of production losses due to missed tolerances on the workpiece.
Thus, square shafts 15 are typically more costly to manufacture and maintain, and can provide undesirable production losses. Round shafts (not shown) are typically less costly, however, the prevention of rotation of a round shaft is typically accomplished by addition of an anti-rotation pin or other mechanism, wherein the anti-rotation pin or mechanism typically adds length to the linear actuator, especially when the linear actuator is fluid-driven, thus requiring some form of a piston and cylinder arrangement. Thus, conventionally, the anti-rotation mechanism is a separate component coupled to an end of a cylindrical piston and cylinder arrangement, wherein the additional length added by the anti-rotation mechanism can be deleterious in certain applications requiring an abbreviated length linear actuator.
Accordingly, a need exists in the art for a reliable, low-maintenance linear actuator that provides accurate positioning of the shaft over a substantially longer period of use than previously achieved. Further, limiting critical tolerances during manufacture of the linear actuator is desired, wherein manufacturing costs can be contained. Such a linear actuator should overcome, or at least minimize, the above-described drawbacks. Preferably, the linear actuator would comprise a simple and economical, yet reliable, device that would accurately position the shaft with a minimum of wear to the linear actuator over its lifetime, while also having less reliance on maintaining numerous critical dimensions during manufacture. Further, the prevention of rotation of the shaft should not significantly add to the overall length of the linear actuator.